Method for forming a paint film and coating obtained by the method

ABSTRACT

Method for forming paint films by the 2-coats/1-bake method, using a clearcoat paint in which the resin component comprises (a) a copolymer of a vinyl monomer mixture containing at least 20 wt % (meth)acrylic acid cycloalkyl esters 40-90 wt %, (b) alkyl-etherified melamine resin 5-50 wt %, and (c) blocked polyisocyanate 5-50 wt %, and a basecoat paint in which the resin component comprises (d) a copolymer of a vinyl monomer mixture containing at least 10% of monomers represented by the general formula ##STR1## (X 1  -X 5  are H or a methyl group, i is 0-2, j is 0 or 1, k is 0-3, m is 0-3, and n is 1-10), 40-80 wt %, (e) alkyl-etherified melamine resin 10-50 wt %, (f) blocked polyisocyanate 0-40 wt %, (g) cellulose type resin 0-20 wt %, and (h) organic crosslinked fine particles 0-30 wt %, and coatings obtained by the said method.

This application is a 371 of PCT/EP93/03035, filed Oct. 30, 1993.

The present invention relates to a novel method for forming paint filmsand to coatings obtained by the method. More specifically, the presentinvention relates to a method for forming topcoat paint films which areparticularly excellent in acid resistance and scratch resistance, andhave excellent flexibility and adherence when recoated, and to coatingsobtained by means of this method for forming paint films.

In recent years, the durability of paint films, and particularly rainstains due to acid rain and scratching due to car-wash brushes or sandparticles thrown up during travelling, have been giving problems as faras automobile paint films are concerned.

Prior known measures to solve such problems include the application ofclear coats using Y-caprolactone-modified acrylic resins and melaminehardeners (Japanese Unexamined Patent [Kokai] 64-66274), for example,for scratch resistance, and the application of clear coats in whichcarboxylic acids and epoxy reaction are used with acrylic resins andmelamine curing agents, for example, for acid resistance (JapaneseUnexamined Patent 4-114069).

However, with the former technique paint films with excellent scratchresistance are obtained, but resistance to acid raid is inadequate; andwith the latter technique paint films with excellent acid resistance areobtained, but scratch resistance is inadequate.

Raising the glass transition temperature of the cured clearcoat paintfilm of the outer surface layer is also known to be an effective methodfor obtaining a higher degree of acid resistance, but this has theproblem that it invites undesirable conditions such as lowering thebending resistance and recoating adhesion of the paint film, as well aslowering scratch resistance.

Therefore, it has been extremely difficult to find a technique givingpaint films which have a balance of high levels of acid resistance andscratch resistance and are also excellent in bending resistance andrecoating adhesion.

Given this situation, the present inventors have made it their aim tooffer a method for forming paint films by the 2-coats/1-bake method inwhich the paint films are provided with a balance of high levels of bothacid resistance and scratch resistance, together with excellent bendingresistance and recoating adhesion, and coatings obtained by this method.

As the result of concerted research to achieve the aforementioned aim,the present inventors discovered that the aim can be achieved bycombining a clearcoat paint in which the resin component comprises acopolymer of a vinyl monomer mixture containing specified quantities of(meth)acrylic acid cycloalkyl esters, alkyl-etherified melamine resinand blocked polyisocyanate compounds, and a basecoat paint in which theresin component comprises a vinyl type copolymer having hydroxyl groupsplaced at positions separated from the main chain, alkyl-etherifiedmelamine resin and possibly blocked polyisocyanate compounds and/orcellulose type resin; and they have perfected the present inventionbased on this insight.

Thus, the present invention offers a method for forming paint filmscharacterised in that, in forming a paint film by coating a basecoatpaint including a colouring and after then coating a clearcoat paintover the top curing the said basecoat and clearcoat simultaneously byheating, a paint in which the resin component comprises (a) a copolymerof a vinyl monomer mixture containing at least 20% by weight ascycloalkyl esters of acrylic acid or methacrylic acid 40-90% by weight,(b) an alkyl-etherified melamine resin 5-50% by weight, and (c) blockedpolyisocyanate compounds 5-50% by weight, is used as the said clearcoatpaint, and a paint in which the resin component comprises (d) acopolymer of a vinyl monomer mixture containing at least 10% by weightas monomers represented by the general formula ##STR2##

(In the formula, X¹ -X⁵, which may be the same or different, are each ahydrogen atom or a methyl group; i is 0 or 1 or 2; j is 0 or 1, k is 0or an integer 1-3; m is 0 or an integer 1-3; and n is an integer 1-10)

40-80% by weight, (e) an alkyl-etherified melamine resin 10-50% byweight, (f) blocked polyisocyanate compounds 0-40% by weight, (g) acellulose type resin 0-20% by weight, and (h) crosslinked organic fineparticles having a particle size of 0.01-5 μm 0-30% by weight, is usedas the said basecoat paint, and coatings obtained by this method.

Below the present invention will be explained in detail. In paint filmsobtained by the method for forming paint films of the present invention,the clearcoat has a high glass transition temperature and excellent acidresistance, and because blocked polyisocyanate compounds are used italso has excellent scratch resistance based on the cohesive force ofurethane bonds. And because in the basecoat a vinyl type copolymerhaving hydroxyl groups positioned separate from the main chain via byflexible atomic groupings is used in combination with a melamine resinand possibly a blocked polyisocyanate curing agent, bending resistanceand recoating adhesion are excellent.

The copolymer of Component (a) used in the clearcoat paint in the methodof the present invention can be obtained by usual free radicalcopolymerization of a (meth)acrylic acid cycloalkyl ester with a vinylmonomer containing hydroxyl groups and/or other vinyl monomer(s). As(meth)acrylic acid cycloalkyl esters here, for example cyclohexyl(meth)acrylate, isobornyl (meth)acrylate, tetracyclo[4,4,0,1²,9,1⁷,10]dodecyl-3-(meth)acrylate, adamantyl (meth)acrylate, t-butylcyclohexyl(meth)acrylate, and 2-(meth)acryloyloxyethylhexahydrophthalic acid,etc., can be cited; 1 of these can be used, or a combination of 2 ormore can be used. The said (meth)acrylic acid cycloalkyl ester needs tobe used at at least 20 wt % in the total vinyl monomer mixture used toobtain Component (a). If the quantity employed is less than 20 wt % theacid resistance of the paint film obtained is inadequate, and thepurpose of the present invention cannot be achieved. Similarly, examplesof the aforementioned vinyl monomers containing hydroxyl groups include(meth)acrylic hydroxyalkyl ethers such as 2-hydroxyethyl (meth)acrylate,2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate andhydroxybutyl (meth)acrylate, etc., monoethers of polyether polyols suchas polyethylene glycol, polypropylene glycol and polybutylene glycol andvinyl monomers containing hydroxyl groups such as 2-hydroxyethyl(meth)acrylate, etc., addition compounds of α-β unsaturated carboxylicacids with monoepoxy compounds such as Cardura E10 (trade name, ShellPetroleum Chemicals) or α-olefin epoxides, addition compounds ofglycidyl (meth)acrylate with monobasic acids such as acetic acid,propionic acid, p-(t-butyl)benzoic acid or fatty acids, monoestercompounds or diester compounds of vinyl monomers having acid anhydridegroups such as maleic anhydride or itaconic anhydride with glycols suchas ethylene glycol, 1,6-hexanediol or neopentyl glycol, etc.,hydroxyalkyl vinyl ethers such as hydroxyethyl vinyl ether, andlactone-modified vinyl monomers made by addition of 1-10 mols oflactones such as ε-caprolactone, β-methyl δ-valerolactone,γ-valerolactone, δ-valerolactone, δ-caprolactone, γ-caprolactone,-propionolactone or γ-butyrolactone to a (meth)acrylic acid hydroxyalkylester such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl(meth)acrylate or hydroxybutyl (meth)acrylate, etc.; 1 of these can beused, or they can be used in combinations of 2 or more.

Moreover, examples of other vinyl monomers include methyl(meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl(meth)acrylate, isobutyl (meth)acrylate, t-butyl (meth)acrylate,2-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, lauryl(meth)acrylate, tridecyl (meth)acrylate, stearyl (meth)acrylate,styrene, -methylstyrene, vinyltoluene, (meth)acrylonitrile,(meth)acrylamide, vinyl acetate, (meth)acrylic acid and glycidyl(meth)acrylate; 1 of these can be used, or 2 or more can be used incombination.

It is desirable that the glass transition temperature of the copolymerof Component (a) be at least 30° C., in order to confer on the finalpaint film the excellent acid resistance desired. The copolymer ofComponent (a) can be obtained by carrying out a copolymerizationreaction according to usual methods using a mixture of theaforementioned vinyl monomers, in the presence of a known initiator suchas for example azobis(isobutyronitrile), benzoyl peroxide, t-butylperbenzoate, t-butyl hydroperoxide, t-butyl octanoate, t-butyl peroxideor cumene hydroperoxide, etc.

The alkyl-etherified melamine resin of Component (b) used in theclearcoat paint of the present invention can be obtained bymethylolation and alkyl etherification of aminotriazine withcyclohexanol or an alkanol of 1-6 carbons, and butyl-etherified melamineresin methyl-etherified melamine resin and butyl/methyl-etherifiedmelamine resin, etc., can be cited. Specific examples ofbutyl-etherified melamine resin include U-Van (trade name, Mitsui ToatsuChemicals), and Super Beckamine (trade name Dainippon Ink & Chemicals),etc.; and specific examples of methyl-etherified melamine resins andbutyl/methyl-etherified melamine resins include Cymel (trade name,Mitsui Cyanamid) and Nikalac (trade name, Sanwa Chemicals), etc.

Examples of blocked polyisocyanate compounds of Component (c) which canbe used in the clearcoat paint in the present invention includenon-yellowing polyisocyanate compounds, and more specificallypolyisocyanate compounds such as hexamethylene diisocyanate,trimethylhexamethylene diisocyanate, isophorone diisocyanate,hydrogenated diphenylmethane diisocyanate and hydrogenated xylylenediisocyanate, etc., and their adducts, biurets and isocyanurates,blocked by normal isocyanate blocking agents such as for example phenoland phenols such as m-cresol, xylenol and thiophenol, etc., alcoholssuch as methanol, ethanol, butanol, 2-ethylhexanol and cyclohexanol,etc., and compounds such as caprolactam, methyl ethyl ketone oxime,ethyl acetoacetate and diethyl malonate, etc., containing activehydrogen.

The proportions of each of the resin components used in the clearcoatpaint in the present invention need to be within the ranges Component(a) 40-90 wt %, Component (b) 5-50 wt % and Component (c) 5-50 wt %,based on the total weight of the resin components. With the saidComponent (a) here at less than 40 wt % the bending resistance of thefinal paint film is inadequate, and if it exceeds 90 wt % scratchresistance is lowered. Similarly, with Component (b) at less than 5 wt %solvent resistance is inadequate, and if it exceeds 50 wt %, acidresistance is lowered. Moreover, with Component (c) at less than 5 wt %scratch resistance is inadequate, and if it exceeds 50 wt % the curedpaint film will be prone to yellowing.

The copolymer of Component (d) used in the basecoat paint in the presentinvention is obtained by copolymerizing a monomer represented by thegeneral formula ##STR3##

(In the formula x¹ -x⁵ i, j, k, m and n have the same meanings aspreviously) with other vinyl monomers. Examples of monomers representedby General Formula [1] here include monoethers of polyether polyols suchas polyethylene glycol, polypropylene glycol, and polybutylene glycol,etc., with vinyl monomers containing hydroxyl groups such as2-hydroxyethyl (meth)acrylate or 2-hydroxypropyl (meth)acrylate, etc.,monoester compounds or diester compounds of vinyl monomers containingacid anhydride groups, such as maleic anhydride and itaconic anhydride,etc., with glycols such as polyethylene gly[co]l, polypropylene glycoland 1,6-hexanediol, etc., and lactone-modified vinyl monomers with 1-10mols of lactone such as ε-caprolactone, β-methyl-δ-valerolactone,γ-valerolactone, δ-valerolactone, -δcaprolactone, γ-caprolactone,β-propionolactone or -γbutyrolactone, etc., added to a vinyl monomercontaining a hydroxyl group, such as 2-hydroxyethyl (meth)acrylate,2-hydroxypropyl (meth)acrylate or hydroxybutyl (meth)acrylate, etc.; 1of these can be used, or they can be used in combinations of 2 or more.

Other vinyl monomers include vinyl monomers which can be used to obtainthe aforementioned Component (a), and 2-hydroxyethyl (meth)acrylate,2-hydroxypopyl (meth)acrylate and hydroxybutyl (meth)acrylate, etc.; 1of these can be used, or a combination of 2 or more can be used.

In the mixture of vinyl monomers used to obtain the copolymer of thesaid Component (d), monomers represented by the aforementioned GeneralFormula [1] need to be used at at least 10 wt %. When this quantity isless than 10 wt %, bending resistance and recoating adhesion are loweredand the purpose of the present invention cannot be achieved. Thecopolymer of the said Component (d), like the copolymer of theaforementioned Component (a), can be obtained by carrying out acopolymerization reaction according to a usual method in the presence ofa known initiator.

The alkyl-etherified melamine resin of Component (e) and the blockedpolyisocyanate compounds of Component (f) used in the basecoat paint inthe present invention can be the same as the alkyl etherified melamineresin of the aforementioned Component (b) and the blocked polyisocyanatecompounds of the aforementioned Component (c). The cellulose type resinof Component (g) is used to facilitate 2-coat/1-bake coating; examplesinclude cellulose acetate/butyrate resins such as CAB-381-0.5, ditto381-2, ditto 531-1 and ditto 551-0.5 (trade names, Eastman-Kodak), etc.

Moreover, the organic crosslinked fine particles of a particle size of0.01-5 μm of Component (h) used in the basecoat paint in the method ofthe present invention are used to facilitate 2-coats/1-bake painting,and fine particles which do not dissolve in organic liquids aredesirable. Such organic crosslinked fine particles can be obtained, forexample, by emulsion polymerizing a vinyl monomer with a crosslinkabledivinyl, trivinyl or tetravinyl monomer in an aqueous medium to form afine particle copolymer, and then removing water by solvent exchange,azeotropic boiling, centrifugal separation, filtration, or a drier,etc., or by carrying out a condensation reaction to copolymerize a vinylmonomer and a crosslinkable vinyl monomer in the presence of adispersing/stabilizing agent in a nonaqueous organic solvent such as analiphatic hydrocarbon, etc., which will dissolve the vinyl monomers butwill not dissolve the polymer, to obtain organic crosslinked fineparticles by the so-called NAD method.

The proportions of each of the resin components used in the basecoatpaint in the method of the present invention, based on the total weightof resin components, need to be in the ranges Component (d) 40-80 wt %,Component (e) 10-50 wt %, Component (f) 0-40 wt %, Component (g) 0-20 wt% and Component (h) 0-30 wt %. With the said Component (d) here at lessthan 40 wt % the bending resistance of the paint film obtained isinadequate, and if it exceeds 80 wt % solvent resistance is lowered.With the said Component (e) at less than 10 wt % solvent resistance isweakened, and if it exceeds 50 wt % bending resistance is lowered.Similarly, if Component (f) exceeds 40 wt % the cured paint film willbecome prone to yellowing.

Moreover, if Component (g) exceeds 20 wt % or Component (h) exceeds 30wt % recoating adhesion will be lowered.

With the method of the present invention it is desirable that the glasstransition temperature of the cured clearcoat film is at least 70° C.,and preferably in the range 70°-150° C., in order to obtain theexcellent acid resistance desired. Bending resistance is lowered if thesaid glass transition temperature exceeds 150° C., so this is notpreferred. In addition to the aforementioned Components (a), (b) and (c)in the clearcoat paint used in the method of the present invention,polymers such as acrylic polymers, polyester polymers, urethanepolymers, non-water-dispersible acrylic polymers and crosslinked polymerparticles, etc., and also additives commonly used in prior paints suchas colourings such as pigments and dyes, etc., pigment dispersants,thickeners, anti-dripping agents levelling agents, curing catalysts,anti-gelling agents, UV protecting agents and free-radical quenchingagents, etc., can be added as required, within ranges that do notadversely prejudice the purpose of the present invention.

Similarly, in addition to the aforementioned Components (d), (e), (f),(g) and (h) in the basecoat paint used in the present invention, variouscolourings, for example metallic pigments such as squamatose metallicpowders such as aluminium, copper, brass, bronze, sheet iron oxide,etc., and mica fragments coated with titanium oxide, etc., colouringpigments such as for example inorganic pigments such as titaniumdioxide, red ochre, yellow ferric oxide and carbon black, etc., andorganic pigments such as phthalocyanin blue, phthalocyanin green andquinacridone red pigments, red azo pigments, perilene red pigments andisoindolinone yellow pigments, etc., can be employed. Moreover,additives generally used in prior paints such as thickeners,anti-dripping agents, levelling agents, curing catalysts, pigmentdispersants, anti-gelling agents, ultraviolet-absorbing agents and freeradical quenching agents, etc., can also be added.

In painting the clearcoat paint and the basecoat paint in the method forforming paint films of the present invention, they can be diluted beforeuse with commonly used paint solvents such as aromatic solvents, estersolvents, ketone solvents and alcohol solvents, etc.

In the present invention, paint films can be formed by 2-coats/1-baketreatment with the same methods as conventionally used, using theaforementioned basecoat paint and clearcoat paint. For example, thebasecoat can be adjusted to a viscosity of the order of 10-30 seconds(Ford Cup No. 4/20° C.), and this can be painted on to an object to bepainted by air spraying, air respraying or electrostatic coating, etc.,to give a cured film thickness of the order of 10-30 μm. Then, afterstanding for several minutes at room temperature, the clearcoat paint,adjusted to a viscosity of the order of 15-50 seconds (Ford Cup No.4/20° C.), can be painted by air spraying, air respraying orelectrostatic coating, etc., to give a cured film thickness of theclearcoat only of the order of 20-50 μm, and the basecoat and clearcoatpaint films can both be cured simultaneously.

A cured paint film can usually be obtained by allowing a setting time ofthe order of 5-30 minutes after coating the clearcoat paint to make thepaint film adequately smooth and allow a certain degree of volatiledispersion of the solvent inside, followed by drying at a temperature of100°-180° C. for about 10-120 minutes.

Next, the present invention will be explained in more detail by means ofpreparation examples, embodiments and comparison examples; however, thepresent invention is in no way limited by these examples.

Preparation example 1

Preparation of Copolymers C1-C4 and B1-B3

The xylene recorded in Table 1 was put into a 4-mouthed flask fittedwith a thermometer, stirrer, reflux cooler and dropping funnel, and thetemperature was raised to 140° C. Then, a mixture of monomers andpolymerization initiators recorded in Table 1 was added dropwise at aneven rate over 2 hours by means of the dropping funnel. After finishingdropwise addition, the contents were held at reflux temperature for 1hour and cooled to 100° C. After cooling to 100° C. a polymerizationinitiator recorded in Table 1 was added dropwise at an even rate over 30minutes. When the temperature was subsequently held at 100° C. for 3hours to complete the polymerization reaction, solutions of CopolymersC1-C4 and B1-B3 with the properties recorded in Table 1 were obtained.

Notes.

1) CHMA: Cyclohexyl methacrylate

2) IBorMA: Isobornyl methacrylate

3) IBorA: Isobornyl acrylate

4) MMA: Methyl methacrylate

5) St: Styrene

6) 2HEMA: 2-Hydroxyethyl methacrylate

7) 4HBA: 4-Hydroxybutyl acrylate

8) FM2: Placcel (trade name, Daicel Chemical Industries; a monomerrepresented by the aforementioned General Formula [1], of 2-hydroxyethylmethacrylate with 2 mols of added Y-caprolactone).

9) PP1000: Blemmer PP1000 (trade name, Nihon Oil & Fats; a monomerrepresented by the aforementioned General Formula [1], of2-hydroxypropyl methacrylate with 4 mols of added propylene oxide)

10) BMA: n-Butyl methacrylate

11) EHA: 2-Ethylhexyl methacrylate

12) AA: Acrylic acid

13) t-BPBz: Polymerization initiator, t-butyl perbenzoate

14) t-BPEH: Polymerization initiator, t-butyl-2-ethylhexanoate

Preparation examples 2-9

Preparation of basecoat paints

Basecoat paints were made by mixing and dispersing the types andquantities of starting materials shown in Table 2 using a dissolver orpaint shaker.

Notes

1) Super Bekamine L-116-70 (trade name, Nihon Ink Kagaku Kogyo KK;isobutylated melamine resin solution, heat residue 70 wt %)

2) Cymel 303 (trade name, Mitsui Cyanamid; methylated melamine resin,heat residue 98 wt %)

3) Yuban 20SE (trade name, Mitsui Toatsu Chemicals; butylated melamineresin, heat residue 60 wt %)

4) Takenate XB-72-G16 (trade name, Takeda Chemical Industries;non-yellowing blocked isocyanate compound solution, heat residue 60 wt%)

5) Coronate 2513 (trade name, Japan Polyurethane Industry; non-yellowingblocked isocyanate compound solution, heat residue 80 wt %)

6) CAB531-1 (trade name, Eastman Kodak; cellulose acetate/butyrateresin), 15 wt % butyl acetate solution

7) CAB381-2 (trade name, Eastman-Kodak; cellulose acetate/butyrateresin), 15% butyl acetate solution

8) Organic crosslinked fine particular polymer nonaqueous suspensionprepared according to Table 5 Preparation Example B1 of JapaneseUnexamined Patent [Kokai] 1-279902 (heat residue 50 wt %, particlecontent 35 wt %)

9) Alupaste 7160N (aluminium pigment product made by Toyo Aluminium;heat residue 65 wt %)

10) Rubicron Red 500RG (trade name of a quinacridone red pigment made byToso KK)

11) Bengala KN-R (trade name of a red ochre made by Toda Industries)

12) Tinuvin 900 (trade name, Ciba-Geigy) 10% xylene solution

13) 20 wt % isopropyl alcohol solution of p-toluene sulphonic acid

14) Modaflo (trade name, Monsanto; levelling agent)

Preparation Examples 10-17

Preparation of clearcoat paints

The types and quantities of different starting materials shown in Table3 were mixed and dispersed using a dissolver to make a clearcoat paint.

Notes)

1)-4), 6), 7), 9), 10) as above

Desmodur BL-3175 (trade name, Sumitomo Bayer Urethane; non-yellowingblocked isocyanate compound solution, heat residue 75 wt %]

Sanol LS292 (trade name, Mitsui) 10% xylene solution

Embodiments 1-7 and Comparison Examples 1-3

Aqua No. 4200 (trade name, Nihon Oil & Fats cationic electrodepositionpaint) was painted electrostatically onto zinc-phosphate-treated softcopper sheets to give a cured film thickness of 20 μm, followed bybaking at 175° C. for 20 minutes. Then "Haiepiko" No. 100 (trade name,Nihon Oil & Fats, midcoat paint) was airspray painted on to give a curedfilm thickness of 40 μm and baked at 140° C. for 30 minutes to obtainthe test pieces.

The basecoat paints of Preparation Examples 2-9 were diluted using athinner (toluene/butyl acetate/isobutyl alcohol=50/30/20 w/w) to 13seconds with Ford Cup No. 4 (20° C.), and the clearcoat paints ofPreparation Examples 10-17 were diluted using a thinner (Solvesso100/n-butyl alcohol=70/30 w/w) to 25 seconds with Ford Cup No. 4 (20°C.).

The different diluted paints obtained were painted onto the above testpieces in the combinations shown in Table 4: painted sheets werefinished by painting on the basecoat paint by air spraying in 2 stageswith an interval of 1 minute 30 seconds to give a cured film thicknessof 15 μm, followed by setting at normal temperature for 3 minutes, andthen painting on the clearcoat paint by air spraying to give a curedfilm thickness of 40 μm, and baking at 140° C. for 30 minutes.

The paint performance tests shown in Table 5 were carried out on each ofthe finished painted sheets obtained. The results are shown in Table 5.

Notes 1)

Glass transition temperature: A free paint film was made and dynamicviscosities were determined with a Rheovibron DDV-II-EA (Orientic); theglass transition temperature was found from the temperature at whichcompound elasticity showed a peak value.

2) Acid resistance: A 0.2 ml spot of 40 wt % aqueous sulphuric acid wasput onto the test piece, and the degree of staining produced afterheating at 60° C. for 15 minutes was assessed visually.

3) Scratch resistance: After coating muddy water (a mixture of JISZ-8901-84 Type 8 dust/water/car windscreen washer liquid=1/10/2 v/v)onto the test piece with a brush, the test piece was washed for 10seconds in a car washer with the brushes rotated at 150 rpm. Afterrepeating the above operation 4 times, the degree of rubbing damage tothe test piece surface was assessed visually.

4) Bending resistance: The state of detachment of the paint film wasassessed visually after testing bending resistance according to JISK-5400 (1990) 8.1 with a core rod diameter of 10 mm.

5) Recoat adhesion: Basecoat paint and clearcoat paint were painted in2-coats/1-bake twice by the painting method described previously (bakingconditions 140° C.×30 minutes twice), and the state of detachment of thepaint film was assessed visually after testing adhesion according to JISK-5400 (1990) 8.5 with a 2-mm grid size.

6) Resistance to petrol: Swelling, discoloration and detachment of thepaint film were assessed visually after immersing the sample for 7 hoursin regular petrol at 20° C.

7) Weather resistance: Gloss, discoloration and staining of the paintfilm were assessed visually after testing the sample for 1500 hours witha sunshine carbon arc lamp (JIS K-5400 (1990) 9.8.1).

From the results above, it is evident that coatings obtained by themethod for forming paint films of the present invention are excellentboth for acid resistance and scratch resistance, and are also excellentfor bending resistance and recoating adhesion.

By contrast, Example 1 acid resistance was weak because in the copolymerof Component (a) used in the clearcoat no cycloalkyl ester of acrylicacid or methacrylic acid was used, and the glass transition temperatureof the cured clearcoat paint film was also less than 70° C.

With Comparison Example 2, acid resistance was weak because in thecopolymer of Component (a) used in the clearcoat no cycloalkyl ester ofacrylic acid or methacrylic acid was used, and the glass transitiontemperature of the cured clearcoat paint film was less than 70° C.Moreover, scratch resistance was weak because no blocked polyisocyanatecompound of Component (c) was used in the clearcoat.

With Comparison Example 3, bending resistance and recoat adhesion wereweak because in the copolymer of Component (d) used in the base coat nomonomer represented by General Formula (1) was used.

With the method for forming paint films of the present invention,because the clearcoat has as paint film properties a high glasstransition temperature and rubber elasticity based on urethane bonding,and the basecoat has soft segments in the crosslinked portion, paintfilms can be offered which have balanced high levels of performance forboth acid-resistance and scratch-resistance, and are also excellent inbending resistance and recoating adhesion.

                  TABLE 1                                                         ______________________________________                                                  Clearcoat     Basecoat                                                        copolymers    copolymers                                                      C1   C2     C3     C4   B1   B2   B3                                ______________________________________                                        Xylene (pts wt)                                                                           94.5   94.5   94.5 94.5 96.0 96.0 96.0                            Components added                                                              dropwise (pts wt)                                                             CHMA.sup.1) 40.0   --     --   --   --   --   --                              IBorMA.sup.2)                                                                             --     25.0   --   --   --   --   --                              IBorA.sup.3)                                                                              --     --     30.0 --   --   --   --                              MMA.sup.4)  19.3   15.2   0.1  30.0 23.3 7.3  29.2                            St.sup.5)   --     --     --   15.0 20.0 20.0 20.0                            2HEMA.sup.6)                                                                              --     23.2   27.8 23.2 --   --   13.9                            4HBA.sup.7) 20.5   --     --   --   --   --   --                              FM2.sup.8)  --     --     --   --   38.3 --   --                              PP1000.sup.9)                                                                             --     --     --   --   --   53.6 --                              BMA.sup.10) 18.9   35.4   40.8 29.3 --   --   --                              EHA.sup.11) --     --     --   1.2  17.2 17.8 35.6                            AA.sup.12)  1.3    1.3    1.3  1.3  1.3  1.3  1.3                             t-BPBz.sup.13)                                                                            2.5    2.5    2.5  2.5  1.0  1.0  1.0                             Added catalyst                                                                (pts wt)                                                                      t-BPEH.sup.14)                                                                            0.2    0.2    0.2  0.2  0.2  0.2  0.2                             Xylene      2.8    2.8    2.8  2.8  2.8  2.8  2.8                             Properties                                                                    Heat residue                                                                              50     50     50   50   50   50   50                              (wt %)                                                                        Gardner     N-O    S      Q-R  S-T  P    U    R-S                             viscosity                                                                     (25° C.)                                                               Glass       40     60     50   60   0    0    0                               transition                                                                    temp. (°C.)                                                            OH value    80     100    120  100  60   80   60                              Acid value  10     10     10   10   10   10   10                              ______________________________________                                    

                                      TABLE 2                                     __________________________________________________________________________                Preparation                                                                         Preparation                                                                          Preparation                                                                         Preparation                                                                          Preparation                                                                         Preparation                                                                          Preparation                                                                         Preparation                      example 2                                                                           example 3                                                                            example 4                                                                           example 5                                                                            example 6                                                                           example 7                                                                            example                                                                             example              __________________________________________________________________________                                                             9                    Proportions included                                                          (parts by weight)                                                             Copolymer B1 solution                                                                     100.0 100.0  --    --     120.0 120.0  140.0 --                   Copolymer B2 solution                                                                     --    --     130.0 160.0  --    --     --    --                   Copolymer B3 solution                                                                     --    --     --    --     --    --     --    130.0                L-116-70.sup.1)                                                                           35.7  21.4   21.4  --     --    --     --    21.4                 Cymel 303.sup.2)                                                                          --    --     --    15.3   --    --     --    --                   Yuban 20SE.sup.3)                                                                         --    --     --    --     50.0  50.0   16.7  --                   Takenate XB-72-G16.sup.4)                                                                 25.0  41.7   --    --     --    --     --    --                   Coronate 2513.sup.5)                                                                      --    --     --    --      6.3  6.3     18.8 --                   CAB 531-1 solution.sup.6)                                                                 66.7  --     --    --     --    --     --    --                   CAB 381-2 solution .sup.7)                                                                --    --     --    33.3   33.3  33.3   --    --                   MG-1 dispersion.sup.8)                                                                    --    28.6   57.1  --     --    --     14.3  57.1                 Aluminium paste.sup.9)                                                                    15.4  15.4   15.4  15.4   15.4  --     --    15.4                 Quinacridone.sup.10)                                                                      --    --     --    --     --    36     36    --                   Red ochre.sup.11)                                                                         --    --     --    --     --    4      4     --                   UV protector solution.sup.12)                                                              5.0   5.0    5.0   5.0    5.0  5.0    5.0    5.0                 PTSA solution.sup.13)                                                                     --    --     --     2.5   --    --     --    --                   Modaflo.sup.14)                                                                            0.3   0.3    0.3   0.3    0.3  0.3    0.3    0.3                 Isobutyl acetate                                                                          20.0  10.0   10.0  10.0   10.0  40.0   40.0  10.0                 n-Butyl alcohol                                                                            5.0   5.0    5.0   5.0    5.0  10.0   10.0   5.0                 __________________________________________________________________________

                                      TABLE 3                                     __________________________________________________________________________                Preparation                                                                         Preparation                                                                          Preparation                                                                         Preparation                                                                         Preparation                                                                          Preparation                                                                         Preparation                                                                         Preparation                       example 10                                                                          example 11                                                                           example 12                                                                          example 13                                                                          example 14                                                                           example 15                                                                          example                                                                             example               __________________________________________________________________________                                                            17                    Proportions included                                                          (parts by weight)                                                             Copolymer C.sup.1 solution                                                                100.0 120.0  --    --    --     --    --    --                    Copolymer C.sup.2 solution                                                                --    --     140.0 160.0 --     --    --    --                    Copolymer C.sup.3 solution                                                                --    --     --    --    140.0  140.0 --    --                    Copolymer C.sup.4 solution                                                                --    --     --    --    --     --    140.0 140.0                 L-116-70.sup.1)                                                                           21.4  50.0   --    --    --     --    --    --                    Cymel 303.sup.2)                                                                          --    --     20.4  15.3  --     --    --    --                    Yuban 20SE.sup.3)                                                                         --    --     --    --    25.0    8.3  25.0  50.0                  Takenate XB-72-G16.sup.4)                                                                 58.3   8.3   --    --    --     --    --    --                    Desmodur BL-3175.sup.5)                                                                   --    --     13.3   6.7  --     --    --    --                    Coronate 2513.sup.6)                                                                      --    --     --    --    18.8   25.0  18.8  --                    UV absorber solution.sup.7)                                                               10.0  10.0   10.0  10.0  10.0   10.0  10.0  10.0                  Photostabilizer solution.sup.8)                                                           10.0  10.0   10.0  10.0  10.0   10.0  10.0  10.0                  PTSA solution.sup.9)                                                                      --    --      2.5   2.5  --     --    --    --                    Modaflo.sup.10)                                                                            0.8   0.8    0.8   0.8   0.8    0.8   0.8   0.8                  Xylene      10.0  10.0   10.0  10.0  10.0   10.0  10.0  10.0                  n-Butyl alcohol                                                                            5.0   5.0    5.0   5.0   5.0    5.0   5.0   5.0                  __________________________________________________________________________

                  TABLE 4                                                         ______________________________________                                                   Basecoat paint                                                                           Clearcoat paint                                         ______________________________________                                        Embodiment 1 Prep. Example 2                                                                            Prep. Example 10                                    Embodiment 2 Prep. Example 3                                                                            Prep. Example 11                                    Embodiment 3 Prep. Example 4                                                                            Prep. Example 12                                    Embodiment 4 Prep. Example 5                                                                            Prep. Example 13                                    Embodiment 5 Prep. Example 6                                                                            Prep. Example 14                                    Embodiment 6 Prep. Example 7                                                                            Prep. Example 15                                    Embodiment 7 Prep. Example 8                                                                            Prep. Example 15                                    Comparison 1 Prep. Example 6                                                                            Prep. Example 16                                    Comparison 2 Prep. Example 6                                                                            Prep. Example 17                                    Comparison 3 Prep. Example 9                                                                            Prep. Example 12                                    ______________________________________                                    

                                      TABLE 5-1                                   __________________________________________________________________________              Embodiment 1                                                                           Embodiment 2                                                                           Embodiment 3                                                                           Embodiment 4                                                                          Embodiment                       __________________________________________________________________________                                                 5                                Clearcoat (wt %)                                                              Component (a)                                                                           Copolymer C1                                                                           Copolymer C1                                                                           Copolymer C2                                                                           Copolymer C2                                                                          Copolymer C3                               (50)     (60)     (70)     (80)    (70)                             Component (b)                                                                           Super Bekamine                                                                         Super Bekamine                                                                         Cymel 303                                                                              Cymel 303                                                                             Yuban 20SE                                 L-116-70 L-116-70 (20)     (15)    (15)                                       (15)     (35)                                                       Component (c)                                                                           Takenate Takenate Desmodur Desmodur                                                                              Coronate 2513                              XB-72-G16                                                                              XB-72-G16                                                                              BL-3175  BL-3175 (15)                                       (35)     (5)      (10)     (10)                                     Basecoat (wt %)                                                               Component (d)                                                                           Copolymer B1                                                                           Copolymer B1                                                                           Copolymer B2                                                                           Copolymer B2                                                                          Copolymer B1                               (50)     (50)     (65)     (80)    (60)                             Component (e)                                                                           Super Bekamine                                                                         Super Bekamine                                                                         Super Bekamine                                                                         Cymel 303                                                                             Yuban 20SE                                 L-116-70 L-116-70 L-116-70 (15)    (30)                                       (25)     (15)     (15)                                              Component (f)                                                                           Takenate Takenate --       --      Coronate 2513                              XB-72-G16                                                                              XB-72-G16                 (5)                                        (15)     (25)                                                       Component (g)                                                                           CAB 531-1                                                                              --       --       CAB381-2                                                                              CAB381-2                                   (10)                       (5)     (5)                              Component (h)                                                                           --       MG-1     MG-1     --      --                                                  (10)     (20)                                              Paint film                                                                    performance                                                                   Glass transition                                                                        75       80       90       85      87                               temperature (°C.).sup.1)                                               Acid resistance.sup.2)                                                                  No abnormality                                                                         No abnormality                                                                         No abnormality                                                                         No abnormality                                                                        No abnormality                   Scratch resistance.sup.3)                                                               Good     Good     Good     Good    Good                             Bending resistance.sup.4)                                                               No abnormality                                                                         No abnormality                                                                         No abnormality                                                                         No abnormality                                                                        No abnormality                   Recoat adhesion.sup.5)                                                                  No abnormality                                                                         No abnormality                                                                         No abnormality                                                                         No abnormality                                                                        No abnormality                   Petrol resistance.sup.6)                                                                No abnormality                                                                         No abnormality                                                                         No abnormality                                                                         No abnormality                                                                        No abnormality                   Weather resistance.sup.7)                                                               No abnormality                                                                         No abnormality                                                                         No abnormality                                                                         No abnormality                                                                        No abnormality                   __________________________________________________________________________

                                      TABLE 5-2                                   __________________________________________________________________________              Embodiment 6                                                                          Embodiment 7                                                                          Comparison 1                                                                          Comparison 2                                                                          Comparison 3                        __________________________________________________________________________    Clearcoat (wt %)                                                              Component (a)                                                                           Copolymer C3                                                                          Copolymer C3                                                                          Copolymer C4                                                                          Copolymer C4                                                                          Copolymer C2                                  (70)    (70)    (70)    (70)    (70)                                Component (b)                                                                           Yuban 20SE                                                                            Yuban 20SE                                                                            Yuban 20SE                                                                            Yuban 20SE                                                                            Cymel 303                                     (5)     (5)     (15)    (30)    (20)                                Component (c)                                                                           Coronate 2513                                                                         Coronate 2513                                                                         Coronate 2513                                                                         --      BL-3175                                       (5)     (5)     (15)            (10)[Desmodur]                      Basecoat (wt %)                                                               Component (d)                                                                           Copolymer B1                                                                          Copolymer B1                                                                          Copolymer B1                                                                          Copolymer B1                                                                          Copolymer B3                                  (60)    (70)    (60)    (60)    (65)                                Component (e)                                                                           Yuban 20SE                                                                            Yuban 20SE                                                                            Yuban 20SE                                                                            Yuban 20SE                                                                            Super Bekamine                                (30)    (10)    (30)    (30)    L-116-70                                                                      (15)                                Component (f)                                                                           Coronate 2513                                                                         Coronate 2513                                                                         Coronate 2513                                                                         Coronate 2513                                                                         --                                            (5)     (15)    (5)     (5)                                         Component (g)                                                                           CAB 381-2                                                                             --      CAB 381-2                                                                             CAB 381-2                                                                             --                                            (5)             (5)     (5)                                         Component (h)                                                                           --      MG-1    --      --      MG-1                                                  (5)                     (20)                                Paint film                                                                    performance                                                                   Glass transition                                                                        83      83      58      65      90                                  temperature (°C.).sup.1)                                               Acid resistance.sup.2)                                                                  No abnormality                                                                        No abnormality                                                                        Marked staining                                                                       Marked staining                                                                       No abnormality                      Scratch resistance.sup.3)                                                               Good    Good    Good    Marked damage                                                                         Good                                Bending resistance.sup.4)                                                               No abnormality                                                                        No abnormality                                                                        No abnormality                                                                        No abnormality                                                                        Paint film detachment               Recoat adhesion.sup.5)                                                                  No abnormality                                                                        No abnormality                                                                        No abnormality                                                                        No abnormality                                                                        Paint film detachment               Petrol resistance.sup.6)                                                                No abnormality                                                                        No abnormality                                                                        No abnormality                                                                        No abnormality                                                                        No abnormality                      Weather resistance.sup.7)                                                               No abnormality                                                                        No abnormality                                                                        No abnormality                                                                        No abnormality                                                                        No abnormality                      __________________________________________________________________________

We claim:
 1. Method for forming a paint film comprising applying to asubstrate to form a coated substrate a basecoat paint including acolouring and then applying a clearcoat paint subsequently curing thebasecoat and clearcoat simultaneously by heating, wherein the clearcoatpaint includes a resin component comprising (a) a copolymer of a vinylmonomer mixture containing at least 20% by weight cycloalkyl esters ofacrylic acid or methacrylic acid 40-90% by weight, (b) analkyl-etherified melamine resin 5-50% by weight, and (c) blockedpolyisocyanate compounds 5-50% by weight, and (a) the basecoat paintincludes a resin component comprising(d) a copolymer of a vinyl monomermixture containing at least 10% by weight monomers represented by thegeneral formula ##STR4## wherein the formula, X¹ -X⁵, which may be thesame or different, are each a hydrogen atom or a methyl group; i is 0 or1 or 2; j is 0 or 1, k is 0 or an integer 1-3; m is 0 or an integer 1-3;and n is an integer 1-10;4- 80% by weight, (e) an alkyl-etherifiedmelamine resin 10-50% by weight, (f) blocked polyisocyanate compounds0-40% by weight, (g) a cellulose resin 0-20% by weight, and (h)crosslinked organic fine particles having a particle size of 0.01-5 μm0-30% by weight.
 2. Method for forming the paint film according to claim1 wherein the glass transition temperature of the cured clearcoat paintfilm is at least 70° C.
 3. The coated substrate obtained by the methodfor forming the paint film of claim
 1. 4. The coated substrate accordingto claim 3 wherein the glass transition temperature of the curedclearcoat paint film is at least 70° C.